The invention relates to an integrated chromatic dispersion compensator for optical signals in optical communication networks, comprising a plurality of cascaded stages of optical dispersion elements arranged in the form of a lattice filter structure.
An integrated chromatic dispersion compensator of this type was described by K. Takiguchi, S. Kawanashi, H. Takara, A. Himeno and K. Hattori in J. Lightwave Technol. Vol. 16, No. 9, 1998, p. 1647-1656.
Optical communication networks have increased in importance since the transmission of large amounts of data over long distances, in particular via the internet, has become popular.
Digital data transfer via optical communication networks is based on electromagnetic waves of a center frequency on the order of 195 THz carrying a modulation which contains the data to be transmitted. The typical bandwidth of such a modulation is about 40 GHz in a 40 Gbit/s DWDM (wavelength division multiplex) system. The light waves are transferred through optical waveguides, such as glass fibers.
However, the effective refractive index of standard waveguide is frequency dependent due to material dispersion and waveguide dispersion. This means that high frequency parts and low frequency parts of the electromagnetic wave propagate through the waveguide at different speeds (group velocity). This effect is called chromatic dispersion and causes time separations between parts of the same data channel. In the example above, a transmission distance of 100 km causes a time separation of about 240 ps between the low frequency part and the high frequency part of the modulated electromagnetic wave. Since the length of one information bit is only about 25 ps, dispersion may damage the information contained in the data channel.
To overcome this problem, optical communication networks are equipped with dispersion compensators. These are intended to undo chromatic dispersion effects.
One type of dispersion compensator is based on chirped fiber Bragg grating (CFBG) as described in the Technical Digest of the Optical Fiber Communication Conference and Exhibit, Mar. 17-22, 2002, Anaheim, Calif., page 577ff; T. Sugihara et al., Paper ThAA2. The electromagnetic wave is sent into fiber with a refractive index grating written into the core region of the fiber. The grating period of the fiber decreases (or increases) along the fiber. The location of reflection (Bragg refection) of a specific frequency part of the wave is dependent on said grating period. Low frequency parts are reflected later (earlier), whereas high frequency parts are reflected earlier (later) and must propagate through the dead end fiber for a longer time. Through Peltier elements exploiting thermal expansion of the material between the mirror planes, CFBG compensators may be adapted to a specific dispersion situation. This involves the optimization of 1 or 2 temperature parameters. However, a CFBG compensator only works for one data channel. Therefore, dispersion compensation for a waveguide that normally transmits a multitude of data channels (typically 32 with a spacing of 100 GHz) requires the separation of these channels into separate fibers and one CFBG compensator component for each channel. This means very costly procedures and constructions, in particular requiring plenty of rag space in central offices due to the large dimensions of one CFBG compensator component.
A second type of dispersion compensator is constructed as an integrated optic dispersion compensator (equalizer) on a planar light wave circuit; see K. Takiguchi et al. It consists of cascaded Mach-Zehnder interferometers (MZI) in a lattice-form optical filter design. In general, an MZI consists of two 3 dB couplers connected via two waveguide arms, one containing a thermo-optic phase shifter and one containing a delay line. In general, the first coupler distributes incoming electromagnetic waves on the two waveguide arms, with the frequency spectrum in the two arms being different. The delay line causes a time delay for the part of the electromagnetic wave in the corresponding arm, whereas the phaseshifter determines the interference situation in the next coupler. Integrated optical dispersion compensators are in principle suitable for compensating the dispersion of a plurality of data channels at a time due to the repetition of the transfer characteristics with the optical frequency. However, in the state of the art there is no method available for adapting an integrated optical dispersion compensator to a varying degree of dispersion.
A dispersion compensator must be adapted to the amount of dispersion of the incoming electromagnetic wave, i.e. to the waveguide. The dispersion situation at the end of a waveguide of a length on the order of some 100 km is not stationary, but varies over time. The most important reason for such a variation is the temperature dependence of the refractive index of the waveguide material. In the above-mentioned example, a temperature change of 30° C. in an 800 km waveguide causes a time separation of about 50 ps/nm. Other problems arise when a dispersion compensator is to be used with different types of waveguides, in particular different lengths and materials. Then a fine tuning of the dispersion compensator in necessary at the installation, but usually cannot be accomplished with satisfactory accuracy for economic reasons.
Therefore, dispersion compensators not capable of dynamically adapting to varying dispersion situations can only accomplish a coarse compensation, with residual chromatic dispersion remaining uncompensated and increasing the bit error rate (BER) of the optical data transfer process.